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<TITLE>A Recap of Manipulators</TITLE>
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<H2>33.1 A Recap of Manipulators</H2>
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<P>We showed examples of manipulators in <A HREF="28-3.html">Section&nbsp;28.3</A>. There we learned that:</P>
<UL>
<LI><P CLASS="LIST">Manipulators are objects that can be inserted into or extracted from a stream.</P></LI>
<LI><P CLASS="LIST">Such insertions and extractions have specific desirable side effects.</P></LI>
</UL>
<P>As a recap, here is a typical example of two manipulators:</P>

<UL><PRE>
std::cout &lt;&lt; std::setw(10) &lt;&lt; 10.55 &lt;&lt; std::endl;
</PRE></UL>
<P>The inserted objects <SAMP>std::setw(10)</SAMP> and <SAMP>std::endl</SAMP> are the manipulators. As its only side effect, the manipulator <SAMP>setw(10)</SAMP> sets the stream's field width to <SAMP>10</SAMP>. Similarly, the manipulator <SAMP>std::endl</SAMP> inserts the end of line character and flushes the output. </P>
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<P>As we mentioned previously, extensibility is a major advantage of iostreams. We've seen in the previous <A HREF="32-3.html">Section&nbsp;32.3</A> how you can implement inserters and extractors for user-defined types that behave like the built-in input and output operations. Additionally, you can add user-defined manipulators that fit seamlessly into the iostreams framework. In this section, we show how to do this.</P>
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<P>First of all, to be extracted or inserted, a manipulator must be an object of a type that we call <SAMP>manipT</SAMP>, for which overloaded versions of the shift operators exist. (Associated with the manipulator type <SAMP>manipT,</SAMP> there is usually a function called <SAMP>f</SAMP><SAMP><SUB>manipT</SUB></SAMP><SAMP>()</SAMP>that we explain in detail later.) Here's the pattern for the manipulator extractor:</P>

<UL><PRE>
template &lt;class charT, class Traits&gt;
std::basic istream&lt;charT,Traits&gt;&amp;
operator&gt;&gt;(std::basic istream&lt;charT,Traits&gt;&amp; istr,
           const manipT&amp; manip)
{   return f<SUB>manipT</SUB>(istr, ...);   }
</PRE></UL>
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<P>With this extractor defined, you can extract a manipulator <SAMP>Manip</SAMP>, which is an object of type <SAMP>manipT</SAMP>, by simply saying:</P>

<UL><PRE>
std::cin &gt;&gt; Manip;
</PRE></UL>
<P>This results in a call to the <SAMP>operator&gt;&gt;()</SAMP> sketched out above. The manipulator inserter is analogous.</P>
<P>A manipulator's side effect is often created by calling an associated function <SAMP>f</SAMP><SAMP><SUB>manipT</SUB></SAMP><SAMP>()</SAMP> that takes a stream and returns the stream. There are several ways to associate the manipulator type <SAMP>manipT</SAMP> to the function <SAMP>f</SAMP><SAMP><SUB>manipT</SUB></SAMP><SAMP>()</SAMP>, which we explore in the following sections. The iostream framework does not specify a way to implement manipulators, but there is a simple way to write your own manipulators. We explain this technique along with other useful approaches.</P>
<P>It turns out that there is a major difference between manipulators with parameters like <SAMP>std::width(10)</SAMP> and manipulators without parameters like <SAMP>std::endl</SAMP>. Let's start with the simpler case of manipulators without parameters.</P>

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